This invention relates in general to sewing machines and in particular to a new and useful sewing machine having a device for tensioning the needle thread.
A similar sewing machine is known for German patent No. 610,638. With this arrangement, a tensioning device for the upper, or needle, thread is used that has a pin mounted in the arm of the sewing machine designed to hold two adjusting nuts, one of which serves to adjust the main tension, the other to adjust the auxiliary tension. Both adjusting nuts act as a common spring. This spring presses a release disc against two gripping discs, whereby the needle thread running between the gripping discs is subjected to the adjusted tension. In order to release the tension, the release disc is separated from the gripping discs by a peg mounted in a hole in the pin.
With the device pursuant to the patent, it is possible to reinforce the adjusted main tension with an auxiliary tension, or to transmit the auxiliary tension to the gripping discs without the main tension. On the other hand, a quick shift between two different tensions is not possible, because the sewing process must be interrupted in order to apply the desired second tension on the needle thread through at least one of the adjusting nuts.
German utility model No. 17 04 798 discloses a tensioning system wherein it is possible to adjust the main and auxiliary tension on the needle thread separately. In order to hold the two tensioning devices for the main and auxiliary tension, two pins are mounted next to one another on the housing of a sewing machine. To adjust the tension, one adjusting nut must be turned for each tensioning device. The adjusted tension is transmitted to the gripping discs via a conical spring and hence to the needle thread. By activating a wedge that can be pressed between the gripping discs and pushes them apart, the auxiliary tension can be released.
With a device like that described in the Utility Model, an adjusted auxiliary tension can be applied or released as needed, but for the purpose two tensioning devices are required per needle thread, so that with machines that have two or more needle threads a clear view of the work is reduced and the operation is complicated by the double number of tensioning devices for each.